1. Field of the Invention
This invention relates to a novel diamino compound and its derivatives. More particularly it relates to a novel diamino compound useful as a raw material for preparing a polyimide which may be employed as an aligning film in a liquid crystal display element exhibiting a high pretilt angle, and to a dinitro compound which may be used and a diol compound as intermediates of the above diamino compound.
2. Description of the Related Art
As to liquid crystal display elements used for watches, electronic calculators, etc., a twisted nematic (hereinafter abbreviated to TN) mode exhibiting a structure having the aligning direction of nematic liquid crystal molecules twisted by 90.degree. between a pair of upper and lower electrode substrates, has now been employed as a main current. However, as to this display mode, when it is applied to a high duty drive, the contrast is inferior and the angle of view is narrow; hence it has been unsatisfactory for improving the display quality and the display surface. Recently, a liquid crystal display device making use of a super-twisted birefringence effect has been announced (T.J. Scheffer and J. Nethring, Appl. Phys. Lett., 45 (10), 1021 (1984)). Since then, a liquid crystal display element making use of a supertwisted nematic mode (hereinafter abbreviated to STN) having the aligning direction of nematic liquid crystal molecules twisted by 180.degree. to 300.degree. between the upper and lower electrode substrates has been developed; thus even in the case of a liquid crystal display element affording a large pictorial surface, those having a satisfactory display quality have come to be obtained. In the case of an aligning film used for such elements, it is necessary to afford a definite angle (hereinafter abbreviated to pretilt angle) between the substrate planes and the liquid crystal molecules, in order to not only align the liquid crystal molecules, but also to improve response properties and ensure the bistability. Further it is preferred that the larger the twist angle, the larger the pretilt angle. Among these elements, in the case of those having a relatively small twist angle (180.degree. to 200.degree. twist), the interface treatment on the electrode substrates is sufficient with a cell provided with an aligning film having an interface of a currently generally employed pretilt angle (abbreviated to .theta.) of 5.degree. or less. However, in the case of those elements using a mode having a twist angle of 200.degree. to 300.degree. wherein they exhibit a better display quality, it is necessary to use an interface of a higher pretilt angle (5.degree.&lt;.theta..ltoreq.30.degree.); thus a liquid crystal display cell provided with an aligning film satisfying such a pretilt angle is necessary.
In the case of currently used aligning films of polyamides for a TN mode, display cells prepared on the commercial scale have a pretilt angle limited to 5.degree..
Polyimide aligning films having a high pretilt angle for a STN mode are also existent, but they still have a problem in the aspect of the stability and reproducibility of a pretilt angle over the total region of the cell substrates having a broad display area. In order to obtain a high pretilt angle with certainty, a thin film formation of SiO.sub.2, etc. by means of oblique vacuum deposition is the best method among those currently carried out. However, such a thin film formation by means of vacuum deposition, when carried out by commercial mass production, is disadvantageous because of the cost of its preparation apparatus. Thus, it has been earnestly desired to obtain an alignment and a high pretilt angle by means of an interface treatment by rubbing a thin film of an organic substance according to the same method as employed in a conventional TN mode to thereby realize the stability and reproducibility of the pretilt angle.
Japanese patent application laid-open No. Sho 61-240223 discloses a liquid crystal display element provided with a liquid crystal aligning film using a polyimide resin having a repetitive unit expressed by the formula ##STR2##
Further, as a raw material for the unit, a concrete example of a diamine expressed by the formula ##STR3## is disclosed therein.
However, the polyimide aligning film obtained using this diamino compound has a drawback that no high pretilt angle is obtained as illustrated in Comparative example mentioned later. Thus, a novel diamino compound used as a raw material for organic aligning films has been desired.